Machines used to wrap and seal articles and packages in thermoplastic film are well known in the art. Two types of machines are commonly referred to as side-sealing and lap-sealing machines. In the typical side-sealing configuration, an article or set of articles travels, typically via a conveyer belt, toward the machine. A sheet of center-folded plastic film, having two layers, is fed from a direction which is preferably perpendicular to the direction of the conveyer. The two layers of the film are then separated such that the article is placed between the lower layer and the upper layer. On one side of the article is the center-fold, while on the other side, there is an open edge where the two layers are not attached. The machine has a side sealing mechanism, which typically comprises several sets of belts to hold and guide the film, a heating/sealing element that fuses or welds the two free layers together and a cutting element that removes the excess material. Thus, as the article passes by the side sealing mechanism, this open edge is sealed by welding the two layers together, the plastic is cut and the waste is removed and discarded. At this point, the plastic film resembles a tube, with openings at both the leading and trailing ends of the article, but sealed along both sides. As the article continues to advance, an end sealing mechanism is then employed to seal the film at the leading end of the article. The article is further advanced and the end sealing mechanism then seals the film at the trailing end of the article.
Incomplete, inconsistent or sloppy welds can be problematic with these types of machines. One specific issue is caused by the height of the article. If the article to be encapsulated has significant height, the film becomes taut when wrapped around the article. This causes the film to exert forces attempting to pull the film toward the article at the time of side sealing. This can lead to incomplete welds. Attempted solutions to this problem include lowering the speed of the belt, or increasing the temperature of the heating/sealing element to attempt to form the weld before the film can be pulled away. However, neither solution has been satisfactory in rectifying the problem.
U.S. Pat. No. 6,526,728 describes a side-sealing machine, including two sets of cooperating pulleys, where each set includes a drive pulley, a tail pulley and a number of idler pulleys. It further discloses that there are two belts associated with each set of pulleys. The outer set of belts travels roughly in the same direction as the flow of plastic, while the inner set diverges so as to keep the waste material separate from the film after cutting. To keep the film from slipping, longitudinal V-ribbed belts are used, with the upper and lower belts offset so that the peak of the V shape in the upper belt fits into the valley of the V shape in the lower belt, and vice versa. As previously stated, this V pattern exists only in the longitudinal direction. This meshing action is designed to prevent slippage of the film, specifically in the transverse direction.
While this meshing action improves welds, inconsistent seals are still problematic. As stated above, the design of the interlocking belts eliminates any slippage in the direction traverse to the direction of film travel. However, it still allows slippage in the direction of travel. This can cause the film to slip and gather near the cutting element, creating sloppy or incomplete welds.
The belts commonly used in side-sealing machines are typically smooth along the edge which interacts with the pulley system. Thus, even with belts in which the V pattern is transverse to the direction of travel, it is also possible for the belts to slip with respect to the pulleys, also causing inferior welds.